VAC Phi 200 - Manual (Page 10)

9
CHECKING OUTPUT TUBE CONDITION
The condition of each individual KT88 output tube should be checked with the built in indicator LED
lights, located behind each KT88.
Proper setting is indicated by an LED that is dark when no music is playing, but lights ‘with the beat’
as music is played. To achieve this setting, place the provided adjustment screwdriver in the bias
adjust opening for the tube to be set. While watching the associated LED, turn the control clockwise
until the LED lights; then turn the control slightly counterclockwise until the LED just goes out. Be
certain to stop the music before making the reading.
Repeat the adjustments if necessary, as the tubes interact slightly.
Do NOT touch the tubes themselves, as they are hot enough to cause serious burns.
NOTE: Some versions of KT88 will evidence a dull orange glow over a small percentage of the
tube’s anode (see photo).
Output Tube Bias Adjustment:
Bias settings it should be checked when you install your amplifier and
approximately once every month thereafter. It must also be adjusted each time
a tube is replaced.
Allow the amplifier to reach normal operating temperature (about ten minutes
of warmup should be sufficient). Mute the preamplifier and set the bias for the
four KT88 tubes by referencing the amber LED lights behind each tube.
Using the supplied tool (or a small flat bladed screwdriver), rotate each control
in turn clockwise until the associated LED just lights, then back up slightly until the
light just goes out.
Since the adjustments can interact slightly, recheck all four tubes.